It depends on the engine really. If it is a older carb engine with a distributor/single coil. you will run into problems sometime with getting it started again due to all the places for water to get in.

If it is a newer engine with coil-on-plug you cang et water intot he spark plus holes and will have running issues if you pressure wash it. It will clear up after a while normally.
You can also cause some problems by pressure washign an alternator, leaking power steering pump, leaking steerign rack, etc.
For the most part it is safe to pressure wash an engine if you use your head. if you keep the nozzle 3 feet or so away at a minimum you shouldnt break anything. The main things to make sure of are that the engine is COLD when you wash it. putting cold water on a hot engine can cause some warpage on exhaust manifold and i have seen cast aluminum parts crack as well.

the best thing i have foudn to clean an engine is Gunk engine degreaser. it is in a spray bottle with an orange cap. you can get it almost anywhere that sells car wash supplies. it is best to get two cans and use ALL of it at oen time. spray it and let it sit. then jsut rinse it off with a water hose.

Oh yah, that thing is gross, get right in there with the pressure washer, good idea, what ever that cone shaped paper thingy is, make sure you hit that REAL good. Listen to Fowl, start with the engine cold, but when you get to that paper thing a ma jig, make sure you start the motor and rev it really high.

I really hope you are kidding, as you obviously know I am, None of the above mentioned methods will make that motor any cleaner then it already is.

Best way to clean off an engine in my opinion is to cover it in scrub n bubbles and lightly rinse off with the hose. Ive done it to my truck for as long as I can remember. Picked that tip up from a friend who used to race.